Animal Species:Ocean Sunfish, Mola mola (Linnaeus, 1758)

The Ocean Sunfish is an unusual looking fish. It doesn't have a caudal fin, instead it has a clavus, which is formed by extensions of the dorsal and anal fin rays, often making it as tall as it is long.

Standard Common Name

Ocean Sunfish

Identification

The Ocean Sunfish is an unusual looking fish. It doesn't have a caudal fin. Instead it has a clavus, which is formed by extensions of the dorsal and anal fin rays. These take the place of a true tail fin which does not form. The clavus is broadly rounded and has low, rounded projections (called ossicles) which make up the margin.

The teeth in each jaw are fused to form a plate, and the mouth is small in comparison to the body size.

Characters that help separate the species from the other four members of the family are the number of ossicles and the presence of a definite line at the posterior end of the body where the denticles on the skin change from extremely coarse to very fine. The clavus (tail frill) of the Ocean Sunfish has 12 fin rays. This is one of the characters that separate it from the other Australian species in the genus, Mola ramsayi, which has 16 fin rays.

The name Mola comes from the Latin word for millstone and refers to the rounded shape of the fish.

Size range

It grows to at least 3.3 m in total length but possibly as large as 4 m.

Similar Species

Mola ramsayi

Distribution

The Ocean Sunfish occurs in temperate marine waters worldwide.

In Australia, it has been recorded from the central coast of New South Wales to Tasmania and west to Mandurah, Western Australia.

The Ocean Sunfish belongs to the family Molidae and is one of three species recorded from New South Wales waters. The other two are the Southern Ocean Sunfish, Mola ramsayi, and the Slender Sunfish, Ranzania laevis. The fourth Australian species is the Sharptail Sunfish, Masturus lanceolatus. It occurs in southern waters of South Australia and Western Australia.

The map below shows the Australian distribution of the species based on public sightings and specimens in Australian Museums. Click on the map for detailed information. Source: Atlas of Living Australia.

Distribution by collection data

Habitat

Ocean Sunfish are usually found in oceanic waters, but occasionally come inshore. Sunfishes in general are often seen at the surface where they may be mistaken for sharks, because of the large dorsal fin.

In their 2010 paper (see References, below) on Mola mola in the north-west Atlantic, Potter and Howell stated that "There was no observed relationship between the amount of time per day that fish spent in cold water (<10 °C) and the amount of time fish spent near the surface (0–6 m), indicating a lack of evidence for M. mola basking at the surface as a mechanism for behavioral thermoregulation." They also stated that "Fish spent greater than 30% of their time in the top 10 m of the water column, and over 80% of time in the top 200 m. The maximum depth recorded by any fish was 844 m."

Feeding and Diet

Ocean Sunfish feed on jellyfish, salps, ctenophores and occassionally small crustaceans and fishes. Recent research states that they feed on hydrozonans("which are small groups of predatory animals related to jellyfish"). The research sheds light on sunfish feeding behaviour, which may also explain why they bask in the sun on the surface. See also Nakamura, Goto and Sato (2015) in references, below.

Other behaviours and adaptations

The Ocean Sunfish swims by moving its dorsal and anal fins back and forth. Both fins are moved in the same direction at the same time. View an Ocean Sunfish swimming in the video below.

Life cycle

Larval ocean sunfish look quite different to adults.

At 2 mm long, Ocean Sunfish larva have a broad body shape. At this small size the Ocean Sunfish has a primordial tail fin, large pectoral fins and body spines. The spines are proportionally large up until a body length of at least 10 mm. At this size, an Ocean Sunfish looks less like the adult fish and more like the pufferfishes and boxfishes to which it is related.

By 2.7 mm long, the spiny body is more compressed than a 2 mm larva but still less compressed than older fish.

By 35 mm in length, the body spines are much less prominent, the body is deeper and more compressed, and the beak-like teeth and clavus have developed. The adult Ocean Sunfish has no visible spines.

Mating and reproduction

Sunfishes are amazingly fecund fishes. A single adult female can produce up to 300 million tiny buoyant eggs. Fertilization occurs when eggs and sperm are shed into the water.

Predators, Parasites and Diseases

Numerous internal and external parasites were found in the September 11, 2002 autopsy. These sea lice Lepeophtheirus nordmanni are on the fish's tongue in the image.

Economic/social impacts

Sunfishes are harmless to people.

In some areas, the Ocean Sunfish is a commercial species that is caught for human consumption.

On 13th October 1998, staff of the Australian Museum were called to examine an Ocean Sunfish, that was found stuck on the bulbous bow of the cement carrier, MV Goliath, as it tied up to the wharf in Sydney (top image). The huge fish, which weighed approximately 1400 kg was removed from the bow of the ship by the Sydney Waterways Authority. The fish became stuck on the bow off Jervis Bay, New South Wales. It caused the speed of the ship to slow from 14 to 11 knots. The skin of the Ocean Sunfish was so rough it wore the ship's paint work back to the bare metal. The fish measured 3.1 m from the tip of the dorsal fin to the tip of the anal fin, and 2.5 m from the tip of the snout to the end of the clavus. Skin samples were taken from above the pectoral fin and near the tail. These are registered in the Australian Museum Ichthyology Collection as AMS I.38997-001 and AMS I.38997-002.

On September 11, 2002, a 1.8 m long Ocean Sunfish Mola mola washed up on Narrabeen Beach, northern Sydney, New South Wales. A team from National Parks and Taronga Zoo euthanased the dying fish and took it to the zoo for autopsy. The autopsy report stated that the fish was dying of starvation. The images were taken during the autopsy by Australian Museum Staff member Kerryn Parkinson. A range of tissue samples from the fish are now stored in the Australian Museum Fish Collection as AMS I.41536.

These Sunfish often meet their end by being struck by a ship. On 18 September 1908, the Steamer Fiona, was 65 km from Sydney when it suffered a ‘violent concussion’. A boat was lowered over the side and the men onboard saw a Sunfish jammed in the framework of the port propeller. The fish was the largest known at the time, measuring ’10 feet, 2 inches’ (3.1 m) in length and ’13 feet, 4 inches’ (4.1 m) in height.

Pope, E.C et al. 2010 The biology and ecology of the ocean sunfish Mola mola: a review of current knowledge and future research perspectives. Reviews in Fish Biology and Fisheries. 20(4): 471-487, DOI: 10.1007/s11160-009-9155-9.

5 comments

Hi Mark. What a fantastic experience. I'm most envious. As you know there are two species of 'normal looking' sunfishes that occur in the area - the Ocean Sunfish and the Southern Ocean Sunfish. Looking from the surface while the fish was swimming in an upright position, I imagine it would be exceedingly difficult to tell them apart. Amongst other things, these fishes eat jellyfishes. Perhaps the fish was looking for a meal near the Yelta. Thank you for documenting your sighting on the Australian Museum website.

Hi Mark
About 1100, Wednesday 6/04/11, this morning we sighted a sunfish swimming along dock 1, port adelaide, south australia. It swam west along the dock past the steam tug yelta. it was swimming at a medium walking pace about 30 cm below the surface. estimated length between 1.5-1.8 miters. We watched it swim past for about 50m before we lost sight of it.
Hope that this info is useful in someway.
Cheers Mark